1. Field of the Invention
The present invention relates to package substrates, and more particularly, to a package substrate having an electrically connecting structure.
2. Description of the Prior Art
There are two types of semiconductor package structure, namely wire-bonding packages and flip-chip packages. In wire-bonding packages, a semiconductor chip is electrically connected to the package substrate by bonding wires. In flip-chip packages, a semiconductor chip is flip-chip mounted on the package substrate with the active surface (of the semiconductor chip) facing downward, and the semiconductor chip is electrically connected to a solder material of the package substrate via a plurality of bumps. Flip-chip packages are lightweight, thin, short, and small, because they do not use space-demanding bonding wires at all, and yet render distance of signal transmission shortened. Another advantage of flip-chip packages is that the underfill provided between the semiconductor chip and the package substrate ensures reliable bonding therebetween.
To allow the semiconductor chip-mounted package substrate of a flip-chip package to be electrically connected to an external electronic device (for example, a printed circuit board), a plurality of solder balls are implanted on the bottom surface of the package substrate.
A solder material is usually formed on electrically connecting pads (chip-mounting area) of a package substrate by a stencil printing technique described below. An insulating protective layer with a plurality of openings therein is formed on a package substrate with a completely laid out circuit. From the openings, a plurality of electrically connecting pads on the package substrate are exposed. A stencil with a plurality of openings therein is disposed on the insulating protective layer of the package substrate. Through the openings of the stencil, a solder pile is formed on the electrically connecting pads, using a squeegee blade or by spraying, as a result of accumulation of solder in the openings and subsequent removal of the stencil. Afterward, the solder pile on the electrically connecting pads is solidified by a reflow process so to form a solder structure.
Referring to FIGS. 1A and 1B, which are cross-sectional views showing a solder material formed on electrically connecting pads of a package substrate according to the prior art, a package substrate substance 10 has at least a surface 10a formed with a plurality of electrically connecting pads 11 thereon, and an insulating protective layer 12 is formed on the surface 10a and the electrically connecting pads 11. The insulating protective layer 12 has a plurality of openings 120 formed therein. The openings 120 correspond in position to the electrically connecting pads 11 so as to expose portions of the electrically connecting pads 11, respectively. A corner C with an angle of 90° approximately is formed between each of the electrically connecting pads 11 and a corresponding one of the openings 120 of the insulating protective layer 12, and thus a solder material 13 formed in the openings 120 of the insulating protective layer 12 is unlikely to be deposited at the corner C. Also, during a reflow process performed on the solder material 13, the corner C with a 90° angle cannot be fully filled with the molten solder material 13 due to cohesion and surface tension thereof, thus causing a gap S (shown in FIG. 1B) to form between the solder material 13 and the insulating protective layer 12. The gap S generates and holds air bubbles readily. As a result, a subsequent process is flawed by unreliability, for example, detachment of the solder material 13.
As mentioned earlier, the corner C between each of the electrically connecting pads 11 and each of the openings 120 of the insulating protective layer 12 cannot be fully filled with the solder material 13, and thus the area of contact between the solder material 13 and each of the electrically connecting pads 11 is unfavorably small, and in consequence the solder material 13, from which a solder structure is going to be made subsequently, is unlikely to be attached to the electrically connecting pads 11 to the detriment of the quality of solder balls and the electrical connection performance of the package substrate.
Bleeding of the solder material 13 during a reflow process is prevented solely by the solder masking characteristics of the insulating protective layer 12. However, a short circuit is readily formed because of formation of a solder bridge between the bled solder material 13 on the adjacent electrically connecting pads of a package substrate having a fine pitch as soon as the solder material 13 turns molten during the reflow process. To solve the problem, the pitch of the solder material 13 has to be widened, which means that the package substrate is no longer fine-pitch, so to speak.
In view of this, an issue that calls for an immediate solution involves eliminating known drawbacks of the prior art, namely, inefficient formation of a solder material, formation of gaps between the solder material successfully formed and the insulating protective layer, poor electrical connection between a solder structure and the package substrate, and bleeding of the solder material in a reflow process.